roman letters vs roman type

roman letters and roman type both are nouns.

Word	Noun	Adjective	Verb	Adverb
roman letters	Yes	No	No	No
roman type	Yes	No	No	No

As nouns, roman letters and roman type are synonyms defined as:

roman letters and roman type: a typeface used in ancient Roman inscriptions

Other synonyms of roman letters include roman, roman print.

roman letters (noun)	roman type (noun)
a typeface used in ancient Roman inscriptions	a typeface used in ancient Roman inscriptions